If you want to open a TIFF image, you have to use pdf_open_image_file(). If you want to open a TIFF image from memory, you'll have to wait until version 5. I gleaned this from the PDFlib folks themselves:

> from rp@pdflib.comThe only way to open TIFF with PDFlib 4 is to open it fromfile. But in PDFlib 5 (http://www.pdflib.com/beta/) we adda virtual file system, so you can easy open a file frommemory.

Nope. Unfortunately PDF supports _nearly_ everything needed
to decode GIFs internally (as with JPEG), but some minor
feature is missing. It seems Adobe also realized this --
they added the missing link in PostScript 3, so chances are
it will appear in PDF 1.4 or something...

Since the PDF_open_image() interface is meant for dumping
the image data to the PDF without any further interpretation,
it can't support GIF.